1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine, and more particularly to a spark plug including an electrode chip joined to a ground electrode at a position opposing a center electrode via a spark discharge gap formed therebetween.
2. Description of the Related Art
A spark plug including an electrode chip jointed to a distal end portion of a ground electrode at a position opposing a center electrode so as to enhance ignitability is known. To decrease wear and tear resulting from spark discharge, a noble metal having a high spark wear resistance (for example, Ir) is used for the electrode chip.
Since the ground electrode is generally formed of an Ni based alloy, the ability to join such noble metal to the ground electrode by welding is poor. In order to enhance the joinability, a spark plug including a molten bond is known (see, for example, U.S. Pat. No. 7,030,544). The molten bond is formed by laser welding around the periphery of an electrode chip (noble metal member). The molten bond reduces a difference in thermal expansion between the ground electrode and the electrode chip, to thereby enhance joinability. In U.S. Pat. No. 7,030,544, in order to increase joinability, a recessed portion is formed on the ground electrode. Further, while an electrode chip is fitted into the recessed portion, a laser beam is applied from a direction perpendicular to the boundary between the ground electrode and the electrode chip, to thereby form the molten bond.
However, in U.S. Pat. No. 7,030,544, the laser beam is applied to a side surface of the ground electrode, but cannot be applied from an extending direction (i.e., a direction opposite the tip end surface) of the ground electrode. Namely, a molten bond is not formed on the side of the electrode chip opposite the tip end surface of the ground electrode. Consequently, upon subjecting the spark plug to cooling and heating cycles in use, it is difficult to sufficiently reduce the thermal expansion difference between the electrode chip and the ground electrode. Thus, the electrode chip may dislodge in a part where no molten bond is formed. Further, the molten bond is formed as an alloy layer of a mixture of a component of the electrode chip and a component of the ground electrode. When the alloy layer contains a relatively large amount of the component of the electrode chip, oxidation corrosion easily occurs in the alloy layer as compared with the electrode chip and the ground electrode. Thus, if an outer surface of the molten bond formed of the alloy layer is exposed to combustion gas, oxidation corrosion is likely to occur, which leads to separation (falling off) of the electrode chip.